Potion, uncommon
When you drink this potion, you gain the "enlarge" effect of the enlarge/reduce spell for 1d4 hours (no concentration required). The red in the potion's liquid continuously expands from a tiny bead to color the clear liquid around it and then contracts. Shaking the bottle fails to interrupt this process.
Notes: Control, Utility, Consumable
What is the second half of this item description about? expanding and contracting red bead?
That's the physical description of what the potion looks like. Purely flavor
I wonder why potion of growth is uncommon and potion of diminution is rare.
what is the cost of this potion?
Does the potion give temporary hitpoints or increase damage?
It grants extra damage as per the enlarge part of enlarge/reduce.
It's easier to expand vs compress
Are there any adverse affects to giving this to animals?
For instance, can Bear, my Goblin Barbarian (Bear Totem), feed a Potion of Growth to his Mastiff, also named Bear, to get better use of the Mounted Combatant feat?
The effects are the same for all creatures (unless they have features/traits preventing it specifically). So feed that to your pet if you want to.
Can this be used on a construct? Specifically a Steel Defender.
RAW, if the Steel Defender is built in such a way that it can drink, I don't see why not.